This application relates generally to motors. The teachings are considered particularly applicable in the field of robotics and active orthotics.
Motors are used in a wide variety of applications. In many applications, including robotics and active orthotics, it may be useful to imitate characteristics similar to human muscles. Such characteristics include, for example, the ability to deliver high torque at a relatively low speed, and to allow free-movement when power is removed, thereby allowing a limb to swing freely during portions of the movement cycle.
With a standard DC motor, torque varies directly in proportion to the motor current. This relationship is expressed as a torque constant, KT, which may be in N-m per amp. The same constant relates voltage to rotation speed. In SI units, KV=KT, which may be in Volts/rad/s. A DC motor is normally designed with a single torque constant. This means the motor operating at a fixed power input cannot dynamically trade off speed for torque. Accordingly, manufacturers typically sell families of motors with different motor constants depending on whether the application needs high torque (high KT) or high speed (low KV). This is a significant drawback for applications that require relatively fast, low torque operations as well as slower, high torque example, imitate the modes of operation of human muscles, which allow the same arm to swat a fly (fast, low torque) and to lift a heavy weight (slow, high torque).
Standard electric motors typically operate at thousands of RPMs, and the range of typical motor constants does not extend down to the point where standard motors can deliver extremely high torque at low speed. In order to provide this capability, a reduction gear must be added to convert the motor's high speed and low torque into the desired low speed and high torque. Current reduction gearing techniques include spur gears, worm gears, pulleys and harmonic drive gears. All of these techniques decrease efficiency and have other undesirable characteristics including the addition of cost, weight, volume, and noise. Also, when an output shaft is driven through a high gear ratio, it is difficult to turn the output shaft when the motor is not powered. The absence of an unpowered free-movement mode is a significant disadvantage in some applications.
Most motors are also inefficient when moving slowly while holding tension against an external load. In order for a slowly moving motor to hold its current position, significant current must be applied to the motor windings and this current results in large power dissipation even though no work is being performed on the output load. A mechanical reduction gear, such as a worm gear, can avoid this power loss when moving slowly, but this type of gearing also makes the free movement mode impossible.